The present invention relates generally to an indirectly-fired gas turbine system and more particularly to such a system wherein water is injected into the compressed air stream upstream of the air heater to increase the operational efficiency and power output of the turbine system. The United States Government has rights in this invention pursuant to the employer-employee relationship of the U.S. Department of Energy and the inventor.
Gas turbine systems generally fall into two classes with one class being of the direct-fired type wherein a fuel is burned in the presence of compressed air to provide high temperature combustion products which form the motive fluid for expansion in the turbine while the other class is of the indirectly-fired type wherein the compressed air is indirectly heated in an air heater for providing the motive fluid for the turbine. In the indirectly-fired systems, residual heat in the turbine exhaust is recoverable in a combustion chamber downstream of the turbine wherein a fuel such as coal, natural gas, fuel gas, or oil is burned to provide a stream of hot combustion gases for use in the air heater or heat exchanger utilized to heat the compressed air stream used as the turbine driving motive fluid. In an indirectly-fired system, the turbine is not contactable by such combustion products so as to provide operational advantages over directly-fired systems especially in the area where the turbine components are subject to considerable degradation by contact with the combustion products. The indirectly-fired gas turbine system is believed to be particularly advantageous for the utilization of coal or a coal product such as fuel gas as a fuel in the combustion chamber since no combustion products enter the turbine circuit.
Gas turbine systems employing indirect cycle operations have not been previously found to be adequately competitive with the turbine systems using direct cycle operations since metallic-type heat exchangers, as previously known and used for indirectly heating the compressed air, limited the heating of the latter to a maximum temperature of about 800.degree. C. At such a relatively low temperature, the motive fluid for driving the turbine could not provide the level of work required in the turbine to efficiently compete with the direct cycle systems. Recent developments in high-temperature heat exchangers have substantially overcome this temperature limiting obstacle of the earlier heat exchangers so that the compressed air charge can be heated to turbine inlet temperatures in excess of 1,000.degree. C. so as to significantly increase the operating efficiency of the indirect cycle. One such heat exchanger which is suitable for heating the compressed air in an indirect cycle is made up of ceramic tubes as described in U.S. Pat. No. 4,332,295, issued Jun. 1, 1982, to P. G. LaHaye et al. By employing an appropriate bundle of such ceramic heat exchange tubes, the high temperature combustion gases resulting from the combustion of a fuel such as coal, preferably in the presence of the warm air from the turbine exhaust, contact the hot ceramic tubes to indirectly heat the compressed air to a turbine inlet temperature in a range of about 1100.degree. to 1260.degree. C. In as much as the water-augmented indirectly-fired gas turbine system of the present invention, as will be described below, will utilize a heat exchanger of high temperature air heating capabilities such as described in the aforementioned patent for heating the compressed air to a turbine inlet temperature greater than about 1000.degree. C., this patent is incorporated herein by reference.
The utilization of high temperature heat exchangers or air heaters has increased the efficiency of indirect cycle turbine systems to levels favorably comparing with and often more efficient than the direct-fired turbine systems. For example, a coal-fired indirect cycle at typical operating pressures and with the compressed air heated to provide a turbine inlet temperature of 1260.degree. C. would provide a calculated net cycle efficiency in the 32 to 37 percentile range. The addition of a steam bottoming cycle would increase the net efficiency of the system to about 44 to 48 percent. On the other hand, a natural gas-fired direct cycle system using conventional temperatures and pressures would typically provide a net operating efficiency of about 31 percent with the steam bottoming cycle increasing the efficiency to about 46 percent.